Calculations on ground and excited state potential energy surfaces of floppy free radicals: HC4H2, HC3NH, and HC3O Original Research Article

Ab initio calculations using the 6-311G(d, p) basis set at the QCISD and CCSD(T) levels were undertaken on the ground and lowest excited electronic states of the isoelectronic carbon-chain free radicals, HC4H2, HC3NH and HC3O. The equilibrium geometry for each molecule is a Cs structure with four electrons in a″ orbitals, and the lowest excited states are characterized by configurations with three, five, and six a″ electrons. The effective vibrational potential energy surfaces of these states along the two lowest frequency in-plane bending coordinates were investigated in order to assess the trend in large-amplitude motions and isomerization across different electronic configurations in a series of isoelectronic molecules. The (4-a″) ground state potential surfaces of HC3NH and HC3O are subject to the Renner-Teller effect, and intersect the (5-a″) excited state surfaces at linear 2π states of at 2300 cm−1 and 8400 cm−1, respectively, above the energy of the equilibrium geometries. The (3-a″) states of HC3O and HC3NH lie 27 000 cm−1 and 24 000 cm−1 above the (4-a″) equilibrium states, respectively, and form Renner-Teller pairs with the (6-a″) tates. Although two minima are found for HC3O and HC3NH on the (3-a″) surface, none of these minima is stable with respect to planarity. The (5-a″) surface of HC4H2 has a single minimum at a C2v structure, which is unstable with respect to the ground state. The (3-a″) surface has two minima, both of which are stable. Equilibrium structures, excitation energies, and potential surface properties for these structures are reported.